The present invention relates generally to an image processing apparatus, and more particularly to an image processing apparatus which is applied to an image processing part of a digital image output unit, such as a copier, a facsimile or a printer, by which pseudo halftone images having an appropriate image quality can be generated from a multilevel tone data by making use of a prescribed image processing technique.
Conventionally, the dither process, which is a prior image processing technique, has been used to generate a halftone data from a multilevel tone data inputted from an original document by a scanner, and convert the halftone image data into a data appropriate for being outputted by an output device, such as a printer, in which only a bilevel image data can be outputted suitably. When the dither process is applied to the image processing, the input multilevel tone data is compared with a predetermined dither matrix pattern so as to determine the value of a bilevel data for each pixel in the output image data. However, the dither process described above has a disadvantage in that a high resolution of a reproduced image is not compatible with a multiple gray level of the output image data, that is, it is inevitable in the case where the dither process is applied that the higher the resolution, the lower is the gray level.
On the other hand, there is another known halftone rendition technique which can provide a reproduced image having a good resolution together with a sufficient gray level. Such a prior halftone rendition technique is generally called the error diffusion method. This conventional error diffusion method has a feature, namely, that an error of optical density of an image produced when a bilevel rendition process is performed is stored in an error buffer memory for use in the bilevel rendition process of pixels adjacent to a pixel being considered so that the optical density of the image from an original document is maintained by the image processing after a multilevel tone process is performed. The multilevel tone process described above is a process that is performed in order to reduce the number of tone levels of input pixels to a number of tone levels of an output image data that can be outputted with a reproduced image having a suitable image quality by the output device including a copier, a facsimile and a printer. Conventionally, a bilevel rendition technique such as the dither process is applied to most of the output devices, because the output devices can suitably output a bilevel image data only. However, the multilevel tone process described above is not limited to the bilevel rendition process, as it can also include several multilevel rendition processes such as two-level, three-level, ... and sixteen-level rendition processes. The conventional error diffusion technique described above is disclosed, for example, in "Digital Halftoning" (the name of a paper contained in a journal published by the Institute of Television Engineers of Japan).
As described above, it is known that the conventional error diffusion process is often applied when a pseudo halftone data is generated from an input multilevel tone data inputted by the scanner. However, when the above conventional error diffusion technique adapted for bilevel rendition is applied without any modification or change to the multilevel tone process so that a processed image data is outputted by an output device, such as a printer, in which a multilevel tone data is assigned to each pixel included in the output image data, there is a problem in that a pseudo outline may sometimes be produced at portions of a reproduced image where the optical density of the reproduced image does not change clearly, or an undesired moire pattern may appear in the reproduced image especially when a screened halftone data is processed. In addition, the conventional error diffusion method adapted primarily for performing a bilevel rendition process has a disadvantage in that it is very difficult to supply a processed image data appropriate for being outputted by a printer in which a multilevel tone data is assigned to each pixel included in the output image data. To eliminate this problem, there is an improved multilevel tone process which does not use the prior error diffusion method for bilevel rendition, and in this multilevel tone process a multilevel rendition is carried out by a corrected data being compared with a plurality of predetermined threshold levels. However, the hardware of an error diffusion part of the image processing apparatus to which such an improved multilevel tone process is applied must be changed in accordance with the particular output device used, since there is a different optimal number of tone levels for each type of output device or printer.